According to the Scherzer theorem, it is not possible to prevent axially chromatic and spherical aberration in rotational symmetrical electron lenses. The aperture aberration dominates and limits the resolution of electron microscopes and microprobes at voltages greater than 50 KV.
The chromatic aberration limits the resolution significantly especially at potential voltages less than 5 KV between the source and the object plane. This low voltage electron microscopy is utilized for controlling the following: microcircuits, the direct investigation of surfaces of nonconductors, biological objects as well as semiconductors having high spatial resolution.
The limitation caused by the chromatic aberration results from the fact that the penetration depth of the incident electrons on the object extends only a few atom layers at acceleration voltages of between 30 and 10.sup.3 V. This leads to the condition that the resolution is essentially limited by the diameter of the electron probe and not by the exiting depth of the secondary electrons.
The wavelength of the incident electrons is significantly less than the obtainable resolution. A rough estimate shows that the chromatic aberration is by far the most limiting and for a resolution of 1 nm at 1 KV, the spherical as well as the chromatic aberration must be eliminated.
Up until now, the electrical and magnetic octupole corrections afford the only possibility for simultaneous correction of the axially chromatic and spherical aberration. In this connection, reference may be made to the article by H. Rose entitled "Abbildungseigenschaften spharisch korrigierter elektronenoptischer Achromate", Optik, Volume 33, pages 1 to 23 (1971) and the further article by H. Rose entitled "Elektronenoptische Aplanate", Optik, Volume 34, pages 285 to 311 (1971). Unfortunately, these known systems all comprise many elements which are difficult to adjust. Furthermore, the elements are very sensitive with respect to mechanical vibrations and the changes of the individual quadrupole fields. This notwithstanding, it could be shown that these filters are in a position to correct electron lenses. Reference may be made to the article by H. Koops, entitled "Erprobung eines chromatisch korrigierten elektronenmikroskopischen Objektives", Optik, Volume 52, pages 1 to 17 (1978/1979) and the article by W. Bernhard entitled "Erprobung eines spharisch und chromatisch korrigierten Elektronenmikroskopes", Optik, Volume 57, pages 73 to 93 (1980).
It is very helpful if the paraxial beam path within the corrector is configured so as to be rotationally symmetrical in order that the number of correcting elements can be reduced thereby simplifying the adjustment. This is, for example, the case in systems which consist of round lenses and sextupoles. These systems can be corrected with respect to the spherical aberration. The article by V. Beck entitled "A hexapole spherical aberration corrector", Optik, Volume 53, pages 241 to 255 (1979) is pertinent as is the article by H. Rose entitled "Correction of Aperture Aberrations in Magnetic Systems with Threefold Symmetry", Nuclear Instruments and Methods, Volume 187, pages 187 to 199 (1981). In this case, the spherical aberration of round lenses is compensated by means of a combined aberration of two spatially separated sextupoles. The system described in the above-cited article entitled "Correction of Aperture Aberrations in Magnetic Systems with Threefold Symmetry" has no errors of the second and fourth order and it is therefore suitable also as a corrector for quiescent-image transmission electron microscopes. Unfortunately, these systems cannot be utilized for correcting the axially chromatic aberration.
An inhomogeneous Wien filter, which is corrected for all geometrical aberrations of the second order, was already suggested in the article by H. Rose entitled "The retarding Wien filter as a high-performance imaging filter", Optik, Volume 77, pages 26 to 34 (1987). In addition to a high dispersion, this filter has an axially chromatic aberration which unfortunately has the same sign as the chromatic aberration of a round lens. A filter of this kind can therefore not be applied to correct the axially chromatic aberration.